Melt the chocolate and butter in a deep container in a water bath on the stove. Then, remove from the stove, add the sugar and stir until you get a homogeneous mixture.
Leave it to cool and add the eggs while stirring nonstop. Then add the flour and vanilla.
Pour the mixture into an oven dish and bake in a preheated 374°F (190 °C) oven.
Once it's baked, flip it over. Decorate if desired or, even better, pour chocolate ganache over it.